Recently, as greenhouse gases have emerged as a serious problem all over the world, vehicle exhaust gas regulations have been enforced. Therefore, the most vehicle industry has been trying on reducing vehicle weight to meet international environmental regulations and to improve fuel efficiency. Thereby, vehicle manufacturers have been developing technologies to reduce the weight of a vehicle frame forming the majority of a vehicle weight.
A vehicle frame includes a Body In White (BIW) corresponding to a vehicle body skeleton and operating parts, such as doors, a hood, a trunk, and the like, and forms the majority of the total vehicle weight. Therefore, in the vehicle industry, vehicle weight reduction effects may be greatly increased using light materials for vehicle frames.
Further, the vehicle frame is an important part having a decisive effect on vehicle performance, such as vehicle stiffness, collision and driving safety, and the like. Therefore, it may be understood that strength enhancement together with weight reduction of the vehicle frame are necessary technologies. For the reasons of coexistence of safety and weight reduction of a vehicle, a high-strength steel pipe has been used as some structural members.
For instance, a steel pipe may be installed within a vehicle door so as to minimize damage to a vehicle frame and to protect passengers when vehicle side collision occurs. Such a steel pipe is referred to as a door impact beam and the door impact beam has been formed of high-strength steel to achieve strength enhancement and weight reduction of the vehicle frame. Further, greater strength of the steel pipe has been acquired using structure control through alloy design and heat treatment of steel during manufacture of the steel pipe.
A conventional high-strength steel pipe has been manufactured through a quenching process by direct injection of cooling water, immediately after heating to a temperature of 910° C. or greater through high-frequency induction heating. By such a process, the conventional steel pipe may be transformed from a ferrite and martensite structure of a strength of 500 to 600 MPa to a martensite single-phase structure of a strength of 1500 to 2000 MPa after quenching. Such a martensite single-phase structure may have high brittleness. Therefore, when a steel pipe is formed of such a structure, brittle fracture may occur in an early stage of collision of the vehicle and thus safety of the vehicle and driver's safety may not be sufficiently acquired. In this case, the steel pipe may not sufficiently serve as a door impact beam formed to minimize damage to a vehicle frame and to protect passengers.
In order to solve the above-described conventional problems, a tempering process in which reheating to a temperature of about 200° C. has been carried out after first heat treatment so as to reduce brittleness is added. However, because manufacturing costs may increase due to construction of additional equipment for the tempering process and increase in process time, the tempering process may not be applied to manufacture of a steel pipe. Therefore, in order to solve these problems, a steel pipe with improved toughness and high strength, to have improved crash performance, has been demanded.
Accordingly, the present invention addresses the above mentioned technical difficulties thereby improving toughness of the steel pipe used as a door impact beam of a vehicle and improving impact absorption performance.